The Challenge of Attaching Dissimilar Materials - Structural Interfaces and Attachments in Biology - page 6

Fig. 1.3 Acoustic microscopy image of a crack initiating from a corner at the interface between a
silicon die and the epoxy underfill in a flip chip package (adapted, with permission, from [ 6 ])
Fig. 1.4 Low-velocity impact damage in a sandwich panel. Left : Facing subject to impact. Right :
facing-core debonding (adapted, with permission, from [ 7 ])
were manufactured from E-glass/vinylester, while the solid polymeric core was
manufactured from Corecell A800. The central region of punctured material is
surrounded by a lighter colored region with an area nearly three times that of the
puncture. This corresponds to a region of debonding between the face sheet and
the core material. As will be discussed in Chap. 3, the opening of a free edge between
the face sheet and the core can introduce stresses sufficiently large to drive a crack
between face sheet and core.
A possible approach to alleviate facing-core debonding in sandwich structures
utilizes graded interfaces, where the volume fractions of facing and core materials
vary over a short distance from pure facing to pure core material. A modification of
this concept is based on using a graded core, where the properties vary throughout
its entire depth to achieve a desired response, while eliminating a sharp property
mismatch along the interface (Fig. 1.5 ). It has been experimentally shown that,
while damage in conventional sandwich structures involves a facing-core
debonding, such a mode can be eliminated through the use of a graded core. For